Material Matters

Bill Hargin

Trace separation; length parallelism; stackup: Does one stand out?

It’s been some time since I’ve seen an article on crosstalk, so I decided to take the opportunity to walk through the subject in a soup-to-nuts overview for those in the PCB design community who may be interested in why crosstalk-savvy PCB designers and hardware engineers use various design rules for controlling crosstalk. In the process of doing so, we’ll identify which design tweaks provide the most leverage for controlling far-end crosstalk.

Crosstalk is unwanted noise generated between signals. It occurs when two or more nets on a PCB are coupled to each other electromagnetically, (even though conductively they are not connected at all). Such coupling can arise any time two nets run next to each other for any significant length. When a signal is driven on one of the lines, the electric and magnetic fields it generates cause an unexpected signal to also appear on the nearby line, as shown in FIGURE 1.

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Read more: The No. 1 Weapon Against Crosstalk

Bill Hargin

Most boards will work just fine. But what if they don’t?

Over the past year, I’ve written a good bit about glass-weave skew (GWS) and next-generation loss requirements, using PCI Express guidelines as a means of tracking what higher frequencies do to eye patterns. This month, we’ll combine important elements of both these technology series, with just a bit of review in order to make this column one that can be read as-is.

The problem with human behavior is many of us wait for some sort of catastrophic event before we course-correct. When should we get serious about glass-weave skew, as opposed to ignoring it, while hoping it doesn’t turn around and bite us at some point in the field? (A near-worst-case scenario.)

When I was marketing signal-integrity software in the 1990s, many engineers would appear on my radar reactively, playing whack-a-mole after spinning multiple prototypes or field failures. Over time, the list of possible causes grew to include crosstalk, loss in all its forms, and eventually power integrity. I’ve noticed many of today’s hardware teams are sort of on cruise control relative to the “fiber-weave effect” as a design concern, so my objective here is to explore the concept of whether designers should worry about it proactively, given the potential impact of seemingly random field failure in production.

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Bill Hargin

Understanding key differences between time and frequency domains.

As March approaches each year, I can count on the bullfrogs around our neighbor’s pond to be out in force, memories of days coaching baseball and softball, my wife’s birthday, and on March 14, “Pi Day,” which has been celebrated by geeks around the globe since 1988. I take the day seriously due to pi’s prevalence in almost every field of science, ranging from astronomy, electromagnetics, physics, to probably several other fields I’m not even thinking about. How did pi find its way into so much science, and what are the implications for electromagnetics?

Before we go into details regarding the time and frequency domains, it’s beneficial to discuss the “unit circle” and radians. A unit circle is simply a circle with a radius of 1 (regardless of units). The circumference of a unit circle is 2π, meaning that one cycle would be 2π, and there would be 2 x 3.14 radians required to complete the circle. This is illustrated in FIGURE 1.

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Bill Hargin

The relative permittivity for FR-4 is just that: relative.

Ed.: This is Part 3 of a three-part series on preparing for next-generation loss requirements.

Last month, in Part 2 of this series, I outlined the means by which insertion-loss requirements are determined. Here, I’ll suggest a better method for obtaining accurate Df numbers without having to go to the trouble of building test boards.

A longtime PCB industry technologist asked me recently, “What’s a good Dk (dielectric constant) number for FR-4?” As the interest in signal integrity (SI) was growing roughly 25 years ago, it started to interest me that many SI practitioners considered FR-4 to have monolithic properties. The question reinforced that some still hold that view. One might say the relative permittivity (ϵr) of FR-4 is 4.3. Someone else would say 4.1. A third says they always use 4.0. As I read up on it, I realized it varies with frequency, resin content (as a percentage, with the inverse being the glass percentage), and the resin system. At lower frequencies, static numbers for vanilla FR-4 were probably fine for impedance calculations and signal integrity, but those days are far behind us at this point.

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